1. Field of the Invention
This invention relates generally to the field of coated filler particles. More particularly, it relates to treated inorganic filler particles for use with polymers and elastomers.
2. Description of the Related Art
Mesoporous inorganic filler particles consisting of aggregated primary particles having a very high hardness, such as fumed and precipitated silica, are widely employed in state-of-the-art elastomeric and polymeric compositions as agents that impart especially high durability. In these compositions, the very small size of the primary particles allows for good optical transparency when the aggregates are sufficiently well-dispersed. On the other hand, particles that are not well-dispersed but remain as aggregates are useful as agents for creating desirable surface textures for improving adhesion or repelling liquids.
The small size of the primary particles also results in a large specific surface area. As a result, the surface characteristics of these particles are paramount in controlling the properties of polymeric and elastomeric formulations into which the particles are incorporated. The surfaces of inorganic particles immediately after production typically contain chemical functionalities that impart undesirable properties to polymeric and elastomeric formulations. Fumed and precipitated silica, for instance, typically feature surfaces with a high concentration of chemically bound silanol groups, in addition to large quantities of physisorbed and chemisorbed water and weakly bound organic impurities. For non-polar polymers and elastomers, and particularly for highly fluorinated polymers and elastomers, the high polarity of such surfaces greatly inhibits the establishment of intimate contact between the inorganic and polymeric or elastomeric components, leading to poor dispersion, mechanical weakness, poor flow properties, and a lack of readily reproducible physical characteristics.
To overcome these limitations, numerous techniques for modifying the surfaces of inorganic particles have been described. For fluorinated polymers and elastomers, a typical approach involves the treatment of silica particles with fluoroalkyl-alkylsilanes. In some instances, a small amount, generally less than 15 parts by weight of fluoroalkyl-alkylsilane per 100 parts by weight of silica, is added. The silane becomes chemically attached to the silica particles, often through formation of a three-dimensional silicate network on the particle surface. These networks minimize the concentration of surface accessible silanols, while also binding fluoroalkyl functional groups to the silane surface, which increases the chemical compatibility of the filler with the fluoropolymer or fluoroelastomer. Although these methods produce inorganic particles that no longer inhibit intimate contact between the filler and the matrix, the limited amount of fluoroalkyl-alkylsilane employed, along with the disorganized nature of the three-dimensional network, results in a surface energy that is typically no lower than around 30 mJ per square meter. For optimal repellency of fats, oils, and greases, a surface energy of 5-30 mJ per square meter is required.
In many cases, fluoroalkyl-alkylsilane treatment of idealized or carefully prepared surfaces of low specific surface area, such as silicon wafers or plate glass, or of high specific surface area (but with a non-discrete aggregated structure), such as a sol-gel, have been utilized. These coated objects, however, cannot be readily deposited onto other substrates by simple techniques such as spraying and thus cannot impart a nanoscale to microscale texture to surfaces not already patterned. In other cases, non-porous silica particles coated with fluoroalkyl-alkylsilanes have been utilized. In these cases, the lack of mesoporosity, as quantified by specific surface area, limits the range of textures that may be imparted to a surface. In particular, textures that are useful for liquid repellency against fluids at pressures beyond a few kPa require roughness at length scales below 100 nm.
In yet other cases, large quantities of fluoroalkyl-alkylsilane have been reportedly mixed with a wide variety of silica particles by non-specific methods, saturating the surfaces with both bound and unbound fluoroalkyl functionality. A more recent approach involves the dispersion of unbound fluorinated organic/inorganic hybrid molecules directly into polymers and elastomers. In such cases, the lack of covalent chemical bonding between the filler and the fluorinated surface treatment causes the treatment to disappear over time due to abrasion or leaching by fluids in contact with the fluorinated polymers or elastomers.
There exists, therefore, a need for a treated filler particle having a well-defined monolayer-like arrangement of fluoroalkyl chains attached to its surface via covalent and thermally stable chemical bonds, such that the surface energy of the particle, for purposes of liquid repellency, is less than 30 mJ per square meter, and such that a formulation incorporating the particles can be coated onto a substrate, with the surface texture of the coating being controlled by conformality with the texture of the particle aggregates so as to further impart desirable liquid repellence characteristics.